This invention relates to a double-tuned output circuit for a linear amplifier used for example in a very high frequency, high band, television transmitter and, more particularly, to a coaxial type primary resonator over-coupled to a secondary coaxial type resonator.
In a VHF television transmitter operating in the high band (174 to 216 MHz) it is necessary that the linear amplifier which is typically the final stage provide a power gain which is essentially constant over any selected six MHz channel.
In order to achieve this condition, it is essential that the output circuit of the amplifier have broad bandwidth. Adequate bandwidth can be obtained through the design of a double-tuned output circuit. Such output circuits at relatively high power output levels (approximately 3 Kw to 25 Kw at peak of sync) incorporate coaxial type primary and secondary circuits. These circuits typically operate in the one-quarter wavelength mode and are tuned to resonance by means of an adjustable shorting plunger between the coaxial elements. Coupling of the primary to the secondary circuit is typically accomplished by means of an adjustable coupling probe capacitor connected between the two circuits.
No great degree of difficulty is encountered achieving the desired coupling of primary and secondary circuits using this technique. At higher power levels (for example 55 Kw), using larger tubes, however, accomplishing capacitive coupling becomes extremely difficult due to larger tube output capacitance and consequently smaller size of the circuit required to resonate the higher tube output capacitance.